Detected by ESET products as Android/DoubleLocker, it is based on the foundations of a particular banking Trojan, renowned for misusing accessibility services of the Android operating system. However, DoubleLocker lacks the functions related to harvesting users’ banking credentials and wiping out their accounts. Instead, it has two other powerful tools for extorting money from its victims.

DoubleLocker can change the device’s PIN, preventing victims from accessing their devices, and also encrypts the data it finds in it – a combination that has not been seen to date in the Android ecosystem.

“Given its banking malware roots, DoubleLocker may well be turned into what can be called ransom-bankers. Two-stage malware that first tries to wipe your bank or PayPal account and subsequently locks your device and data to request a ransom…

Speculation aside, we spotted a test version of such a ransom-banker in the wild as long ago as May, 2017,” comments Lukáš Štefanko, the ESET malware researcher who discovered DoubleLocker.

Distribution

DoubleLocker spreads in the very same way as its banking parent does. It is distributed mostly as a fake Adobe Flash Player through compromised websites.

Once launched, the app requests activation of the malware’s accessibility service, named “Google Play Service”. After the malware obtains the accessibility permissions, it uses them to activate device administrator rights and set itself as the default Home application, in both cases without the user’s consent.

“Setting itself as a default home app – a launcher – is a trick that improves the malware’s persistence. Whenever the user clicks on the Home button, the ransomware gets activated and the device gets locked again. Thanks to using the accessibility service, the user doesn’t know that they launched malware by hitting Home,” explains Lukáš Štefanko.

Locking both device and data

DoubleLocker, once executed on the device, creates two reasons for the victims to pay.

First, it changes the device’s PIN, effectively blocking the victim from using it. The new PIN is set to a random value which is neither stored on the device nor sent anywhere, so it’s impossible for the user or a security expert to recover it. After the ransom is paid, the attacker can remotely reset the PIN and unlock the device.

The ransom has been set to 0.0130 BTC (approximately USD 54 at time of writing) and the message highlights that it must be paid within 24 hours. However, if the ransom is not paid, the data will remain encrypted and will not be deleted.

Figure 1: Encrypted files on a device infected with DoubleLocker

Figure 2: DoubleLocker ransom message

How to get rid of it?

In the ransom note, the user is warned against removing or otherwise blocking DoubleLocker: “Without [the software], you will never be able to get your original files back”. To prevent unwanted removal of the “software”, the crooks even recommend disabling the user’s antivirus software.

“Such advice is irrelevant: all those with a quality security solution installed on their devices are safe from DoubleLocker,” comments Lukáš Štefanko.

Lukáš Štefanko

For users who want to have their device cleaned of the DoubleLocker ransomware, Lukáš Štefanko recommends the following:

For devices that are not rooted and that don’t have a mobile device management solution installed capable of resetting the PIN, the only way to remove the PIN lock screen is via a factory reset.

If the device is rooted then the user can connect to the device by ADB and remove the file where the PIN is stored. For this to work, the device needs to have debugging enabled (Settings -> Developer options -> USB Debugging).

The PIN or password lock screen will be removed and the user can access the device. Then, working in safe mode, the user can deactivate device administrator rights for the malware and uninstall it. In some cases, a device reboot is needed.

“DoubleLocker serves as just another reason for mobile users to have a quality security solution installed, and to backup their data on a regular basis,” concludes Lukáš Štefanko.

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